The present invention is a process for continuously reducing the amount of low molecular weight cyclic organosiloxane in a recirculating process stream. The process comprises washing a process stream in a wash step to reduce chloride content of the process stream, distilling the process stream into a fraction comprising low molecular weight cyclic organosiloxanes and an inert solvent and a high-boiling fraction comprising linear organosiloxanes and high molecular weight cyclic organosiloxanes and reequilibrating the low-boiling fraction in the presence of a reequilibration catalyst to form a reequilibration mixture comprising high molecular weight cyclic organosiloxanes and the inert solvent, and recycling the reequilibration mixture to the washing step.
Dimethyldichlorosilane generally contains small amount of heptane as a by-product of the direct process. When the dimethyldichlorosilane is hydrolyzed to form polysiloxanes the by-product heptane remains as an unwanted odor causing contaminant. The heptane is subsequently removed by passing the hydrolyzate through a distillation column where the hydrolyzate is recovered as a high-boiling bottom stream and a low-boiling overhead stream comprising heptane and lower boiling siloxanes are recycled back into the process. The low boiling siloxanes which consists mostly of hexamethylcyclotrisiloxane is recycled back into the process and as a consequence builds up over time to as much as one-quarter to one-third of the recycle stream. As the hexamethylcyclotrisiloxane content of the recycle stream increases, it significantly impacts the capacity of the column used to remove heptane from the hydrolyzate. As a result of hexamethylcyclotrisiloxane build-up, additional capital is required to make the column large enough to handle the recycled hexamethylcyclotrisiloxane. When the recycle streams' hexamethylcyclotrisiloxane concentration becomes too great, the recycle stream must be removed from the process for disposal as a hazardous waste. Therefore, a process is needed to remove hexamethylcyclotrisiloxane from the recycle stream, while allowing the heptane to be recycled.
Haines et al., U.S. Pat. No. 5,395,956, describe a process for preparing cyclic organohydrogensiloxanes by contacting an organohydrogendichlorosilane with about a stoichiometric equivalent of water to form a hydrolyzate. The hydrolyzate is diluted in an inert solvent and contacted with an acidic rearrangement catalyst to effect formation of cyclic organohydrogensiloxanes. The cyclic organohydrogensiloxanes are separated from the inert solvent and linear organohydrogensiloxanes and the linear organohydrogensiloxanes are then recycled to the process for further contact with the acidic rearrangement catalyst.
Miller et al., U.S. Pat. No. 3,714,213, describe a process for preparing cyclic methylhydrogensiloxanes by contacting linear methylhydrogen siloxanes with an acid catalyst absorbed on a carrier. The process requires the presence of high molecular weight chain termination groups. The yield of tetramethylcyclotetrasiloxane is reported to be about 73 percent.
Crivello et al., U.S. Pat. No. 4,895,967, describe a method for making cyclic organohydrogensiloxanes by contacting a linear organohydrogensiloxane with a heated bed of a cracking catalyst at reduced pressure. The resulting volatile cyclic organohydrogensiloxane is recovered. A typical yield for the method is reported to be about 85 percent.
The present inventors have discovered that hexamethylcyclotrisiloxane can be removed from a process recycle stream to eliminate hexamethylcyclotrisiloxane build-up and the solvent recycled back to the process. The present invention provides a process for continuously removing hexamethylcyclotrisiloxane from a recirculating process stream and passing it through a reequilibration reactor containing a catalyst to reequilibrate the hexamethylcyclotrisiloxane to higher molecular weight cyclic organosiloxanes.